Recently, with the developments in information society, liquid crystal displays having a large screen and a large capacity have been widely used. Among these displays, simple-matrix-type liquid crystal displays, which have a simple panel construction and are advantageous in terms of costs, are extensively adopted.
Conventionally, a 1/M-duty simple-matrix-type liquid crystal display with N.times.M (width.times.length) dots, shown in FIG. 5, is provided with a liquid crystal display panel 21, a signal-side driver 22 connected to the signal electrodes of the liquid crystal display panel 21, a scanning-side driver 23 connected to the scanning electrodes of the liquid crystal display panel 21, a display-data/timing-generation circuit 24, and a power-source circuit 25 that generates bias voltages of V0 to V5 for use in liquid-crystal driving.
The bias voltages V0 to V5 from the power-source circuit 25 are respectively supplied to transmission gates 22a and 23a (hereinafter, referred to as TGs) in the signal-side driver 22 and the scanning-side driver 23. Further, in the signal-side driver 22, the following signals, released from the display-data/timing-generation circuit 24, are supplied to respective circuits: a display-data signal DATA and a shift-clock signal SCK are supplied to a shift register 22c; a scanning clock signal LP is supplied to a latch circuit 22b; and ac-conversion signals FR are supplied to the TGs 22a. In the scanning-side driver 23, the following signals, released from the display-data/timing-generation circuit 24, are supplied to respective circuits: a scanning-start signal FLM and the scanning clock signal LP are supplied to a shift register 23b; and the ac-conversion signals FR are supplied to the TGs 23a.
When these signals are supplied to the signal-side driver 22 as described above, the TGs 22a release signal voltages Xn in response to the ac-conversion signals FR and the display-data signal DATA, as is shown in the following truth table in Table 1.
TABLE 1 ______________________________________ FR DATA Xn ______________________________________ 0 0 V2 1 0 V3 0 1 V0 1 1 V5 ______________________________________
Further, in the scanning-side driver 23, the TGs 23a release scanning voltages Ym in response to the ac-conversion signals FR and the scanning-start signal FLM supplied thereto, as is shown in the following truth table in Table 2.
TABLE 2 ______________________________________ FR FLM Ym ______________________________________ 0 0 V1 1 0 V4 0 1 V5 1 1 V0 ______________________________________
When the signal voltages Xn and the scanning voltages Ym are applied to the respective electrodes, liquid crystal cells, each located at an intersection between the signal-side electrode and the scanning-side electrode, are subjected to the application of driving voltages, each of which corresponds to a difference between the two voltages Xn and Ym.
In accordance with the voltage-averaging method that is known as a driving method used for obtaining an optimal visual discernibility in the above-mentioned simple-matrix-type liquid displays, waveforms of optimal driving voltages, which are used for displaying a black pattern of longitudinal lines in the white background as shown in FIG. 6, are shown in FIGS. 7(a) through 7(e). Moreover, waveforms of optical driving voltages, which are used for displaying a white pattern of longitudinal lines in the black background as shown in FIG. 8, are shown in FIGS. 9(a) through 9(e).
In the above-mentioned figures, FIGS. 7(a) and 9(a) represent the ac-conversion signals to be applied to the respective TGs 22a and 23a; solid lines in FIGS. 7(b) and 9(b) represent waveforms of voltages to be applied to the scanning-side electrodes of Y2, while broken lines therein represent waveforms of voltages to be applied to the signal-side electrodes of X2; solid lines in FIGS. 7(c) and 9(c) represent waveforms of voltages to be applied to the scanning-side electrodes of Y2, while broken lines therein represent waveforms of voltages to be applied to the signal-side electrodes of X3; FIGS. 7(d) and 9(d) represent waveforms of driving voltages of the (X2, Y2) element; and FIGS. 7(e) and 9(e) represent waveforms of driving voltages of the (X3, Y2) element. Moreover, in FIG. 6 and FIG. 8, the elements that are indicated by white circles and white squares are on-state elements for white display, and the elements that are indicated by black circles and crosses are off-state elements for black display.
Here, the driving voltage, which is applied across each signal-side electrode and each scanning-side electrode, is subjected to a polarity inversion for each scanning operation corresponding to a predetermined number of lines that is substantially smaller than the number of scanning lines M (in this case, each scanning operation corresponds to 13 lines); thus, the number of switchovers of the driving voltage is not completely dependent on the display pattern.
At this time, assuming that the waveform of the driving voltage of the (X2, Y2) element shown in FIG. 7(d) is the same as the waveform of the driving voltage of the (X3, Y2) element shown in FIG. 7(e) in an ideal operation, the effective voltage in each element is equal to the on-state voltage (white) that is represented by the following equation: ##EQU1##
Further, assuming that the waveform of the driving voltage of the (X2, Y2) element shown in FIG. 9(b) is the same as the waveform of the driving voltage of the (X3, Y2) element shown in FIG. 9(c), the effective voltage in each element is equal to the off-state voltage (black) that is represented by the following equation: ##EQU2##
Here, in the above-mentioned equations (1) and (2), Vop represents a voltage corresponding to the difference between the bias voltages V0 and V5; M represents the number of scanning lines=1/duty ratio. Further, A represents a bias coefficient by which a maximum value of V.sub.ON /V.sub.OFF is obtained when a=M.sup.1/2 +1.
However, in an actual operation in a conventional liquid crystal display, the waveforms of the driving voltage that are obtained upon displaying a black pattern of longitudinal lines in the white background are indicated by FIGS. 10(a) through 10(e). Accordingly, in the display pattern shown in FIG. 6, the portions that are indicated by the white squares and that are located on the same signal line as the longitudinal line in the pattern have brightness that is different from the brightness of the other background (indicated by white circles in the drawing). Moreover, in an actual operation, the waveforms of the driving voltage that are obtained upon displaying a white pattern of longitudinal lines in the black background are indicated by FIG. 11. Accordingly, in the display pattern shown in FIG. 8, the portions that are indicated by the crosses have brightness that is different from the brightness of the other background (indicated by black circles in the drawing). This phenomenon, wherein elements having brightness different from that of the background appear on the same signal line as the longitudinal line in the pattern, is referred to as a tailing phenomenon.
As indicated by portions enclosed by circles in FIGS. 10 and 11, the tailing phenomenon is caused by distortions in the waveform of the driving voltage that occur on the scanning lines when the polarity is inverted. In other words, in the display pattern of FIG. 6, due to these distortions in the waveform of the driving voltage, the effective voltages of the (X2, Y2) element and other elements in the background (indicated by white circles) become smaller than those obtained by the equation (1), while the effective voltages of the (X3, Y2) element and other elements in the background (indicated by white squares), which are located on the same signal line as the longitudinal line in the pattern, become greater than those obtained by the equation (1).
Moreover, in the display pattern of FIG. 8, due to these distortions in the waveform of the driving voltage, the effective voltages of the (X2, Y2) element and other elements in the background (indicated by black circles) become smaller than those obtained by the equation (2), while the effective voltages of the (X3, Y2) element and other elements in the background (indicated by crosses), which are located on the same signal line as the longitudinal line in the pattern, become greater than those obtained by the equation (2). In display elements of the negative type wherein on-state elements are displayed as white color, the transmittance commonly becomes higher as the effective voltage increases; therefore, the transmittance of each element is represented by: white square&gt;white circle, and cross&gt;black circle. This phenomenon is recognized as the tailing phenomenon.
The tailing phenomenon, which occurs as described above, is called crosstalk. The crosstalk gives rise to a serious problem to be addressed in the simple-matrix-type liquid crystal display since it extremely lowers the picture quality.
Referring to FIGS. 12 and 13, the following description will discuss a mechanism as to how the distortions occur in the waveform in the voltage to be applied to the scanning lines, upon inversion of the polarity.
In CR-load models as shown in FIGS. 12(a) and 12(b), it is conventionally well known that when the voltage to be applied to one terminal (A) of C is switched from +VB to -VB or from -VB to +VB, a differential waveform, indicated by each equation in each direction in the drawing, is exerted in the other terminal (B) of C due to the transient phenomenon.
Assuming that the V1 and V4 levels (voltages on the scanning lines that are not selected) are the relative ground level (0 V) and that the signal-line side corresponds to the input terminal, the circuit network, which is made in the liquid crystal display and which starts from the signal-side driver 22 and reaches the V1 and V4 lines (the ground level) through the display elements (C) and the scanning-line electrodes resistors together with the ON resistors (R) in the scanning-side drivers 23, is identical to the models shown in FIGS. 12(a) and 12(b).
FIGS. 13(a) and 13(b) show circuit network models, each of which shows some of the elements on the Y2 line in the case of displaying a black pattern in the longitudinal lines in the white background and a differential waveform that is exerted on the Y2 line upon inversion of the polarity. These differential waveforms correspond to the portions enclosed by the circles in FIG. 10. Further, the same explanation is given as to the distortions that are caused upon inversion of the polarity in the case when a white pattern in the longitudinal lines is displayed in the black background.
In the models of FIGS. 12(a) and 12(b), the differential waveforms, which have different directions due to the different switching directions, assume analogous waveforms. Therefore, in the models having a plurality of parallel capacity loads as shown in FIGS. 13(a) and 13(b), supposing that Vop and R are constant, the voltage on the scanning-line side is VY, and the voltage on the signal-line side is VX; it is found that the difference C.sub.X between the combined capacity value C.sub.ON of the elements that vary from VY&lt;VX to VY&gt;VX and the combined capacity value C.sub.OFF of the elements that vary from VY&gt;VX to VY&lt;VX will constitute a factor that determines the effective voltage and direction of the differential waveform.
Here, supposing that the capacity equivalent to one dot of the display element is represented by C, C.sub.X =(N-2).multidot.C holds in the case of the above-mentioned display pattern, since C.sub.ON =(N-1).multidot.C, as well as C.sub.OFF =C, holds. Therefore, in conventional liquid crystal displays, crosstalk tends to occur in such a display pattern as C.sub.X has a great value.